Experiments investigate the early development of knowledge of the immediately surrounding world. Three series of experiments focus on human infants? developing perception and understanding of objects, investigating how infants perceive visible objects, represent the existence, location, and behavior of hidden objects, and learn about objects in the categories of animals, tools, and food. Three further series of experiments focus on infants? developing sensitivity to number, investigating how well infants discriminate among numerosities in visual-spatial arrays and in visual and auditory event sequences, whether they detect numerical correspondences both across auditory and visual modalities and across temporal and spatial formats, and whether they are sensitive to numerical order. Three classes of methods are used to assess infants? perceptual and cognitive development. First, preferential looking and head-turning methods focus on infants? tendency to attend longer to events that are novel or to objects that are self-propelled. Second, reaching methods focus on infants? tendency to reach predictively for moving objects and to reach preferentially for objects that have been observed to exhibit a particular behavior or function. Third, methods for recording high-density event-related potentials following the introduction of a novel object or numerosity serve to investigate both the time course and the developing cerebral mechanisms of object and number representation. The long-term goals of this proposal are to discover core human conceptions of the world through studies of their origins in infancy, to chart the role of these core systems in the development of cognitive skills, and to explore the biological mechanisms that support these systems. An understanding of object and numerical cognition in infancy promises to shed light on the structure and acquisition of knowledge more generally, to aid efforts to foster children?s formal and informal learning of science and mathematics, and to contribute to the detection and treatment of children with developmental disabilities such as attention disorders and calculation deficits.